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参数资料
| 型号： | LT3742EUF#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 15/26页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 24-QFN |
| 标准包装： | 2,500 |
| 系列： | PolyPhase® |
| PWM 型： | 电流模式 |
| 输出数： | 2 |
| 频率 - 最大： | 560kHz |
| 占空比： | 100% |
| 电源电压： | 4 V ~ 30 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 24-WFQFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
LT3742
APPLICATIONS INFORMATION
? OUT
? V + V D ?
P LOSS ≈ ?
? I OUT R DS(ON) ? ρ T ??
( 2 ? V )
? I OUT ? C RSS ? f
(Q G ). A few simple guidelines will make the selection
process easier.
The maximum drain current must be higher than the
maximum rated current, I RATED , calculated on the previous
page. Note that the I D specification is largely temperature
dependent (lower I D at higher ambient temperatures), so
most data sheets provide a graph or table of I D versus
temperature to show this.
Ensure that the V DS breakdown voltage is greater than the
maximum input voltage and that the V GS breakdown volt-
age is 8V or greater. The peak-to-peak gate drive for each
MOSFET is ~7V, so also ensure that the device chosen will
be fully enhanced with a V GS of 7V. This may preclude the
use of some MOSFETs with a 20V V GS rating, as some have
too high of a threshold voltage. A good rule of thumb is that
the maximum threshold voltage should be V GS(TH)(MAX) ≤
3V. 4.5V MOSFETs will work as well.
Power losses in the N-channel MOSFET come from two
main sources: the on-resistance, R DS(ON) , and the reverse
transfer capacitance, C RSS . The on-resistance causes
ohmic losses (I 2 R DS(ON) ) which typically dominate at
input voltages below ~15V. The reverse transfer capaci-
tance results in transition losses which typically dominate
for input voltages above ~15V. At higher input voltages,
transition losses rapidly increase to the point that the use
of a higher R DS(ON) device with lower C RSS will actually
0.7
0.6
provide higher efficiency. The power loss in the MOSFET
can be approximated by:
P LOSS = ( ohmic loss ) + ( transition loss )
2
? V IN + V D ?
2
IN
where f is the switching frequency (500kHz) and ρ T is a
normalizing term to account for the on-resistance change
due to temperature. For a maximum ambient temperature
of 70°C, using ρ T ≈ 1.3 is a reasonable choice.
The trade-off in R DS(ON) and C RSS can easily be seen in an
example using real MOSFET values. To generate a 3.3V,
3A (10W) output, consider two typical N-channel power
MOSFETs, both rated at V DS = 30V and both available in
the same SO-8 package, but having ~5x differences in
on-resistance and reverse transfer capacitance:
M1: I D = 11.5A, V GS = 12V, R DS(ON) = 10m Ω , C RSS = 230pF
M2: I D = 6.5A, V GS = 20V, R DS(ON) = 50m Ω , C RSS = 45pF
Power loss is calculated for both devices over a wide input
voltage range (4V ≤ V IN ≤ 30V), and shown in Figure 6 (as
a percentage of the 10W total power). Note that while the
low R DS(ON) device power loss is 5 × lower at low input
0.7
0.6
0.5
0.4
TOTAL =
OHMIC + TRANSITION
0.5
0.4
0.3
0.2
0.1
TRANSITION
OHMIC
0.3
0.2
0.1
TOTAL =
OHMIC + TRANSITION
OHMIC
TRANSITION
0
0
5
10
15
20
25
30
0
0
5
10
15
20
25
30
INPUT VOLTAGE (V)
3742 F06a
Figure 6a. Power Loss Example for M1 (10m Ω , 230pF)
INPUT VOLTAGE (V)
3742 F06b
Figure 6b. Power Loss Example for M2 (50m, 45pF)
3742fa
15
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